Hydrogenation 3-alkyne

The alkyne hydrogenation reaction has been explored extensively by the Hoffmann-La Roche pharmaceutical company, where it is used in the commercial synthesis of vitamin A. The cis isomer of vitamin A produced on hydrogenation is converted to the trans isomer by heating. 

Terminal alkynes are weakly acidic. The alkyne hydrogen can be removed by a strong base such as Na+ NH2 to yield an acetylide anion. An acetylide... 

The used Pd/ACF catalyst shows a higher selectivity than the fresh Lindlar catalyst, for example, 94 1% versus 89 + 2%, respectively, at 90% conversion. The higher yield of 1-hexene is 87 + 2% with the used catalyst versus 82 + 3% of the Lindlar in a 1.3-fold shorter reaction time. Higher catalyst activity and selectivity is attributed to Pd size and monodispersity. Alkynes hydrogenation is structure-sensitive. The highest catalytic activity and alkene selectivity are observed with Pd dispersions <20% [26]. This indicates the importance of the Pd size control during the catalyst preparation. This can be achieved via the modified ME technique. 

The reverse ME technique provides an easy route to obtain monodispersed metal nanoparticles of the defined size. To prepare supported catalyst, metal nanoparticles are first purified from the ME components (liquid phase and excess of surfactant) while retaining their size and monodispersity and then deposited on a structured support. Due to the size control, the synthesized material exhibits high catalytic activity and selectivity in alkyne hydrogenation. Structured support allows suitable catalyst handling and reuse. The method of the catalyst preparation is not difficult and is recommended for the... 

However if the cw-2-pentene isomer or the 1-pentene isomer is added first, the rate of alkyne hydrogenation is unaffected. The iraui -molecule acts as a poison for the alkyne hydrogenation whereas the cis- and 1-pentene isomers have no effect. 

Muetterties has suggested that the dimeric hydride [RhH(P OiPr 3)2]2 catalyzes alkene and alkyne hydrogenation via dinuclear intermediates [91]. However, no kinetic evidence has been reported to prove the integrity of the catalysts during the reactions. On the other hand, studies of the kinetics of the hydrogenation of cyclohexene catalyzed by the heterodinuclear complexes [H(CO) (PPh3)2Ru((u-bim)M(diene)] (M = Rh, Ir bim=2,2 -biimidazolate) suggested that the full catalytic cycle involves dinuclear intermediates [92]. 

Oxidative addition of molecular hydrogen was considered to be involved in the alkyne hydrogenations catalyzed by [Pd(Ar-bian)(dmf)] complexes (4 in Scheme 4.4) [41, 42]. Although the mechanism was not completely addressed, 4 was considered to be the pre-catalyst, the real catalyst most likely being the [Pd(Ar-bian)(alkyne)] complex 18 in Scheme 4.11. Alkyne complex 18 was then invoked to undergo oxidative addition of H2 followed by insertion/elimination or pairwise transfer of hydrogen atoms, giving rise to the alkene-complex 19. 

The hydrogenation of alkynes is a very interesting reaction, since the selectivity toward the partially or the fully reduced product allows the in-situ comparison of the ability of a catalyst to reduce C=C versus C=C bonds. This is perhaps the area in which duster catalysis has been most extensively developed, as recently reviewed by Cabeza [27], Adams and Captain [4], and Dyson [28]. A good number of metal clusters have been employed as catalyst precursors in alkyne hydrogenation, the majority of them containing ruthenium. 

The use of dispersed or immobilized transition metals as catalysts for partial hydrogenation reactions of alkynes has been widely studied. Traditionally, alkyne hydrogenations for the preparation of fine chemicals and biologically active compounds were only performed with heterogeneous catalysts [80-82]. Palladium is the most selective metal catalyst for the semihydrogenation of mono-substituted acetylenes and for the transformation of alkynes to ds-alkenes. Commonly, such selectivity is due to stronger chemisorption of the triple bond on the active center. 

Colloidal catalysts in alkyne hydrogenation are widely used in conventional solvents, but their reactivity and high efficiency were very attractive for application in scC02. This method, which is based on colloidal catalyst dispersed in scC02, yields products of high purity at very high reactions rates. Bimetallic Pd/Au nanoparticles (Pd exclusively at the surface, while Au forms the cores) embedded in block copolymer micelles of polystyrene-block-poly-4-vinylpyridine... 

According to the hydrogen pressure and substrate/Pd ratio, a TOF up to 4 x 106 hr1 was observed for the hydrogenation of 1-hexyne, this being the highest TOF ever reported for alkyne hydrogenation. 

In less-coordinating solvents such as dichloromethane or benzene, most of the cationic rhodium catalysts [Rh(nbd)(PR3)n]+A (19) are less effective as alkyne hydrogenation catalysts [21, 27]. However, in such solvents, a few related cationic and neutral rhodium complexes can efficiently hydrogenate 1-alkynes to the corresponding alkene [27-29]. A kinetic study revealed that a different mechanism operates in dichloromethane, since the rate law for the hydrogenation of phenyl acetylene by [Rh(nbd)(PPh3)2]+BF4 is given by r=k[catalyst][alkyne][pH2]2 [29]. 

Reaction of acetylenic complexes with triosmium dodecacarbonyl leads to a variety of products involving one, two, or three acetylenic units. As with ruthenium, for the monosubstituted alkynes, hydrogen transfer can occur to the metal cluster. Thus, Os3(CO)12 and phenyl-acetylene (L) yield, in refluxing benzene, the derivatives Os3(CO)10L, Os3(CO)10L2, Os3(CO)9L, and HOs3(CO)9(L-H). The general chemistry is summarized in Scheme 2 (131). 

The Dihydrido Iridium Triisopropylphosphine Complex [lrH2(NCMe)3(P Pr3)]BF4 as Alkyne Hydrogenation Catalysts... 

The alkyne hydrogenation catalyst [Rh(7-SPh-8-Ph-7,8-C2B9Hio)(COD)j, 95, (and derivatives thereof) shows dynamic behavior at ambient temperature (Figure 1.23)... 

The use of modifiers in controlling the selectivity in liquid phase alkyne hydrogenation has been studied. Modifiers that are more strongly bound than the intermediate alkene inhibit hydrogenation and isomerization giving high stereo- and chemo-selectivity. One modifier increased the rate of alkyne hydrogenation... 

Scheme 17. DFT-derived mechanism for the alkyne hydrogenation by a neutral palladium(0)-bisphospine complex. ...

Mechanistic aspects of alkene hydrogenation are by now known in considerable detail, but far less information is available on the mechanistic course of the alkyne hydrogenation reaction.160 In general, data on the hydrogenation of alkynes are rather limited.24... 

It is clear that all three types of selectivity are relevant to catalytic hydrogenation reactions and from a consideration of the reaction scheme for alkyne hydrogenation (Fig. 4), it can be deduced that all three factors may be operative simultaneously. Clearly, the selectivity for the formation of the alkene relative to alkane will depend upon a number of factors. If both the alkene and the alkane are formed during one residence of the parent molecule on the surface, the selectivity will depend upon the relative values of k, and k2 (Type II selectivity) and upon the ratio kjk4 (Type II selectivity). Since both of these depend upon the specific properties of the catalyst, they have been termed the mechanistic selectivity factor [38], Once the alkene is produced, the system contains another potential adsorbate and Type I selectivity must be taken into account. It... 

The effects of hydrogen on the infrared spectra of adsorbed acetylene together with evidence from mechanistic studies of alkyne hydrogenation has led to the general conclusion that the acetylenic species active in hydrogenation is associatively bonded to the surface. However, as with monoolefins, there is still doubt as to the precise formulation of the surface—alkyne bonding. In the early work [156], it was assumed that the associatively adsorbed complex was adequately represented as a di-a-bonded olefin, which adopted a cis-configuration. 

This postulate has several implications regarding the mechanism of alkyne hydrogenation these will be discussed in Sect. 4.3. It should be noted, however, that there is as yet little or no direct evidence for structure L, although analogous structures are known to exist with organometallic complexes [161], Such a structure is also consistent with the positive surface potentials observed for acetylene adsorption on evaporated nickel films [88]. 

In the ensuing discussion, it will be assumed that structure L is the relevant species in alkyne hydrogenation, and that the catalytically active adsorbed state of an alkadiene can be represented as a jr-olefin complex in which either one or both olefinic bonds interact with the surface. 

The selectivity values observed in the first stage of the reaction, summarised in Table 23, show similar trends from metal to metal to those observed in alkyne hydrogenation. Again, palladium is almost completely... 

Early attempts to establish the existence or otherwise of a geometric factor were based upon the assumption that the surfaces of metal particles consisted of extensive arrays of atoms arranged in well-defined low index planes the optimum metal—metal distances for the strain-free adsorption of the reactant hydrocarbon were calculated [84,157]. As noted in Sect. 4.2 (p. 50) such an approach led to the conclusion that only certain crystal planes should be active in alkene and alkyne hydrogenation... 

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